1. Field of the Invention
The present invention relates to a solid oxide fuel cell which has high output performance in the operating temperature of from 600° C. to 800° C. by effectively preventing influence of reaction between respective layers in solid oxide fuel cells of electrode supporting type having a lanthanum-gallate oxide in a solid electrolyte layer.
2. Description of the Related Art
Recently, with a view to lowering the operating temperature of the solid oxide fuel cell to 600° C. to 800° C., research on a low temperature operating type solid oxide fuel cell is energetically carried out. A lanthanum-gallate oxide is proposed as the low temperature operating solid oxide fuel cell (for example, see patent reference 1 and patent reference 2). However, the lanthanum-gallate oxide has high reactivity to another material so as to react to an air electrode or a fuel electrode thereby forming a reaction generating phase of high resistance, so that power generating capacity may be decreased. Especially, in the case of the solid oxide fuel cell of electrode supporting type in which the air electrode or the fuel electrode functions as a support and a solid electrolyte layer is formed as thin as possible, it is required to be sintered at a certain degree of high temperature in order for forming a gas-tight solid electrolyte layer. Therefore, there are such disadvantages that the reaction generating phase of high resistance is formed on the interface between the support and the solid electrolyte layer and that metallic components contained in the support diffuse into the solid electrolyte layer so as to reduce an oxygen ionic transference number to short-circuit between the fuel electrode and the air electrode, thus resulting in substantial decrease of the electric power generating capacity.
In view of these disadvantages, in relation to the solid oxide fuel cell in which the fuel electrode serves as the support, there is proposed the art to form a layer comprising La0.45Ce0.55O2 as a reaction control layer between the fuel electrode and the solid electrolyte layer of the lanthanum-gallate oxide (for example, see non-patent reference 1). However, La0.45Ce0.55O2 is a material of extremely low sintering property, so that it is difficult to densify the reaction control layer. As a result, the solid electrolyte layer comprised of the fuel electrode and the lanthanum-gallate oxide reacts through pores of the reaction control layer whereby the reaction phase of high resistance is formed on the interface. On the other hand, in order to prevent the short-circuit between the electrodes based on diffusion of the metallic components contained in the support, it is required to thicken the solid electrolyte layer comprised of the lanthanum-gallate oxide, thus incurring increase in resistance loss of the solid electrolyte layer so as to decrease the electric power generating capacity.
Further, when a cell is produced at such a high temperature (for example, about 1,600° C.) that the reaction control layer is formed in fully densified state, even if the reaction control layer is provided, the metallic components or the like contained in the support and the lanthanum-gallate oxide are subject to reaction to each other through a gaseous phase, while it is difficult to secure porosity of the support, so that the high power generating capacity may not be obtained.
On the other hand, in relation to the solid oxide fuel cell in which the fuel electrode serves as the support, there is proposed the art to form a layer comprising a cerium-containing oxide represented by Ce1-zLnzO2 (0.05≦z≦0.3), whose porosity is not more than 25%, as the reaction control layer between the fuel electrode and the solid electrolyte layer (for example, see patent reference 3). However, when the solid electrolyte layer comprises the lanthanum-gallate oxide, the reactivity to the proposed cerium-containing oxide is elevated so as to have the reaction generating phase of high resistance represented, for example, by SrLaGa3O7 or the like formed on an interface between a lanthanum-gallate layer and the layer of the cerium-containing oxide, so that power generating capacity expected from the physical properties of material itself may not be obtained.
Patent reference 1: Japanese patent application publication No. 2002-15756 (Pages 1-9, FIGS. 1-9).
Patent reference 2: Japanese patent application publication No. 11-335164 (Pages 1-12, FIGS. 1-12).
Patent reference 3: Japanese patent application publication No. 2003-173802 (Pages 1-7, Table 1).
Non-patent reference 1: Electrochemical and Solid-State Letters, 7(5) A105-A107(2004)